Refactor relying_party_key into Warden::WebAuthn::RackHelpers

[tcannonfodder]

As part of the discussion for devise-passkeys, it became clear that Warden::WebAuthn::StrategyHelpers was being included in places that extend beyond its intended purpose. (see: ruby-passkeys/devise-passkeys#25 (comment))

A major cause of that was the relying_party_key method, which devise-passkeys was using to add the relying party to the request ENV.

In order to keep the two gems separated cleanly (to reduce conflicts like this), we need to:

• Break out the relying_party_key into a new Warden::WebAuthn::RackHelpers module
• Auto-include that module as part of Warden::WebAuthn::StrategyHelpers to achieve the same goal of a centrally located default key